The purpose of these studies is to better understand the relationship between CD8+ T-cell responses, viral replication and CD4+ T-cell depletion in gut-associated lymphoid tissue (GALT), and to clarify the ability of GALT T-cells to perform antiviral immune effector functions. We have developed methods to assess the phenotype and function of antigen-specific T-cells freshly isolated from human rectal tissue. For SPECIFICAIM 1, we will assess the frequency, breadth and functional heterogeneity of HIV-specific CDS+ T-cells in GALT and blood. We hypothesize that the frequency and range of specificities of antigen-specific CD8+ T-cells will be greater in mucosal tissues than in blood. Blood and rectal biopsy samples will be obtained from(i) a cross-sectional study of 30 HIV-infected men and women and 10 controls and (ii) a longitudinal study of20 HIV-infected individuals. For SPECIFIC AIM 2, we will assess the effects of HAART on viral load and antigen-specific CD4+ and CD8+ T-cell responses in GALT. We predict a general decline in the magnitude of HIV-specific CD8+ T-cell responses concurrent with decreased tissue viral load during HAART, although some individuals may harbor residual virus and HIV-specific CD8+ T-cells in GALT. We predict that reduced viral load and CD4+ T-cell repopulation will provide partial reconstitution of CD4+ T-cell responses to non-HIV antigens, and may lead to increased cytokine production by HIV-specific CD8+ T-cells in GALT. For these studies, we will perform longitudinal sampling over a two-year period of 20 individuals initiating HAART. For SPECIFIC AIM 3, we will assess the cytotoxic effector functions of antigen-specific CD8+T-cells in GALT. Preliminary results suggest that (i) HIV-specific CTL in blood express low levels of perforin, (ii) perforin expression by all CD8+ T-cells is decreased in GALT relative to blood, and (iii) granzyme A and perforin are differentially expressed. We hypothesize that CD8+ T-cells in GALT may be functionally impaired. We will compare the cytotoxic function of antigen-specific CD8+ T-cells in blood and GALT of HIV-infected individuals and healthy controls. We will also test alternate hypotheses to explain the apparent low levels of perforin expression in GALT.